Method of forming electric insulating films on al - containing silicon steel sheet and surface-coated al-containing silicon steel sheet



July 28,' 1970 T K YAMAMQTO ET AL 3,522,108

METHOD OF FORMING ELECTRIC INSULATING FILMS ON A] -GONTAINING SILICONSTEEL SHEET AND SURFACE-COATED I Al-CONTAINING SILICON STEEL SHEET FiledMarch 17, 1967 Al- CONTAINING SILICON STEEL SHEET GLASSY FILM HAVINGHIGH ELECTRIC INSULATING PROPERTY INVENTORS TAKAAKI YAMAMOTO KANEOAKANUMA BY WWWZ, fldgm ATTORNEYS United States Patent Office 3,522,108METHOD OF FORMING ELECTRIC INSULATING FILMS ON Al-CONTAINING SILICONSTEEL SHEET AND SURFACE-COATED Al-CONTAIN- ING SILICON STEEL SHEETTakaaki Yamamoto and Kaneo Akanuma, Kitakyushu, Japan, assignors toNippon Steel Corporation, Tokyo,

Japan Filed Mar. 17, 1967, Ser. No. 624,068 Claims priority, applicationJapan, Mar. 18, 1966, 41/ 16,902 Int. Cl. C23f 7/04 US. Cl. 148-6 7Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method offorming a glassy film having a high electric insulation adhesion, spacefactor and heat resistance on the surface of a steel sheet in a processfor producing an oriented silicon steel sheet containing Al as aconstituent.

An Al-containing oriented steel sheet (the term oriented shall includedsingle oriented and double oriented hereinafter) is generally obtainedby subjecting a hot-rolled plate or strip to a step including one ormore cold-rollings and, if required, one or more annealings to obtain asteel sheet of final gauge, then subjecting the so-obtained sheet to acontinuous short time annealing (simultaneously decarburizing when sorequired) and subsequently to a final-annealing (box annealing) at ahigh temperature of about 1200 C.

The annealing separator, that is, a film forming agent to be used in thebox annealing is preferably a material which can immediately act as aseparator during the final annealing and combines with the surface oxidelayer to form a highly adhesive glassy film high in electric insulation,value, space factor and heat resistance, because the surface film for amaterial to be used for laminated sheets or wound cores for use inelectric devices is required to have such high adhesion that the filmwill not peel off when subjected to a processing, high space factor andsuch high heat resistance that the characteristics of the film will notbe deteriorated when subjected to high temperature stress-relievingannealing. Such material which can be used as an annealing separator anda film forming agent are known, for example MgO+SiO (U.S.P 2,354,123),Mg(OH) +Ca(OH) (U.S.P. 2,492,682) and oxides of alkaline earth metals(U.S.P. 2,533,351).

According to known electric insulating film forming methods, when Si isselectively oxidized during the decarburizing annealing in a wetreducing atmosphere so that SiO will be formed on the surface of a steelsheet and then the steel sheet is coated on the surface with asuspension consisting of MgO(Mg(OI-I) )H O, is dried and is then finalannealed at a high temperature in a reducing atmosphere, MgO or the likewill prevent the sticking between the steel sheets and will act as anannealing separator and at the same time a part of it will react withthe above mentioned SiO to form a glassy electric insulating film.

3,522,108 Patented July 28, 1970 The present inventors have found that ahighly adhesive glassy film high in the electric insulation value, spacefactor and heat resistance can be obtained even with a known annealingseparator, but have also found that with a known annealing separator, nohighly adhesive film high in electric insulation value can be obtainedon some electric steel sheets. That is to say, it has been found that,when more than 0.01% of sol. Al is contained in a hot-rolled siliconsteel sheet, even if the steel sheet is annealed in a decarburizingatmosphere for a short time, then is coated with the above mentionedconventional annealing separator in a decarburizing atmosphere, is driedand is final annealed by using dry H or a mixture gas of H and N as inthe conventional practice, no highly adhesive glassy film high inelectric insulation value will be obtained. This is understood to bebecause, if A1 is contained in a silicon steel sheet, the Al will bepreferentially oxidized to produce A1 0 on the surface of the steelsheet, while it is being annealed, consequently the reaction whichoccurs in a conventional silicon steel sheet containing no Al in whichthe SiO will react with MgO to produce a favorable glassy film (forexample, in U.S.P. 2,533,331) is interrupted. Further an attempt wasmade to add SiO into MgO in an annealing separator, but no highlyadhesive glassy film was obtained thereby.

There has been already disclosed a method of producing an orientedsilicon steel sheet from a rolled silicon steel sheet containing, forexample 0.01 to 0.09 wt. percent sol. Al. As a result of research on theannealing separator (film forming agent) most adapted to obtain afavorable glassy film in the final annealing in a process for producingan oriented electric steel sheet from a silicon containing Al, thepresent inventors have discovered that a mixture of MgO and MnO isexcellent as an annealing separator.

The present invention is characterized by applying an inorganic oxidefilm forming agent consisting of a solution containing an Mg compoundand an Mn compound mixed so that the weight ratio of MgO to MnO will be98:2 to 20:80 in the coating slurry or in the film after being dried toan annealing separator in the final annealing in a process for producingoriented silicon steel sheet from a hot-rolled silicon steel plate orsteel strip containing 0.01 to 0.09 wt. percent sol. Al so that asurface layer containing A1 0 produced during the short time annealingor the final annealing and said film forming agent will react with eachother to form a glassy electric insulating film.

The Mg compound or Mn compound so called here designates a compoundwhich will become MgO or MnO when heated, such as its oxide, peroxide,hydroxide or carbonate.

The fact that the A1 0 which is to react with the inorganic oxide filmforming agent containing the Mg compound and Mn compound, is obtainedspecifically by the oxidization of Al contained in the silicon steelinstead of being in the form of an added mixing agent is a feature ofthe present invention. When a silicon steel sheet is merely coated onthe surface a mixture of MgO, MnO and A1 0 and is annealed at a hightemperature, no uniform smooth electric insulating film will beobtained.

The formation of a surface layer containing A1 0 on the suiface of anoriented silicon steel sheet containing Al as one of constituents andthe reaction of this surface layer with an inorganic oxide film formingagent containing an Mg compound and an Mn compound are carried outindustrially in such an annealing step as short time annealing orbox-annealing used in a process for produc ing silcon steel sheets.

That is to say, when a silicon steel plate containing, for example,0.010 to 0.090 wt. percent Al and 2.0 to 4.0 wt. percent Si is annealedfor a short time after being cold-rolled so that a surface layercontaining A1 will be produced on the surface, is coated on the surfacewith a suspension in which are mixed, for example, MaO, MnO and H 0 sothat the coating of the annealing is present after the drying in anamount of from 2 to 10 g./m. or preferably 4 to 8 g./m. is dried and isthen box annealed for to 30 hours in a reducing atmosphere, for example,at 1000 to 1200 C., a part of the MgO and MnO will react with thesurface of the steel sheet to produce a glassy film. Needless to say, itis not always necessary to produce a necessary and sufficient amount ofA1 0 during the annealing for a short time. Even when it is produced byproperly selecting the atmosphere of the final annealing or is producedin a required amount in both the short time annealing and finalannealing and is then made to react with MgO-MnO the object of thepresent invention will be able to be attained. Such a glassy film ishighly adhesive and has a high electric insu lation value and spacefactor and its chemical composition is of an Al O -SiO -FeO-MgO-MnOseries when a Mn compound is to be used. Further, when the annealingseparator remaining after the final annealing is analyzed, it can beseen that the content of the Mn compound has decreased and that of Mn inthe steel has somewhat increased. It is thus presumed that the reactiontherein has contributed to the formation of a highly adhesive glassyfilm and other precedingly enumerated properties. When the Mg compoundand Mn compound are, however, mixed so that the weight ratio of MgO:MnOin the separator after the drying is more than 98 of MgO and less than 2of MnO, a highly adhesive and electric insulating film is not obtained.Further, even if the Mg compound and Mn compound are so mixed that theweight ratio of MgOzMnO in the separator after the drying is less than20 of MgO and more than 80 of MnO, the obtained film after the annealingbecomes so rough that the space factor is reduced and the adhesion isimpaired. The mixing weight ratio of MgO to MnO effective for theproduction of a glassy film is in the range of 98:2 to 20:80.Specifically, in the range of 70:30 to 90:10, a favorable result can beobtained. For the Mg compound there can be used MgO and for the Mncompound manganese oxides and manganese hydroxides in such various formsas Mg(OH) Mn O MnO-OH and Mn(OH) can be used; also compounds which willbecome MgO and MnO when heated can be employed. It is considered thatoxygen liberated from Mn0 by the thermodecomposition during theannealing or water produced by the reaction of this liberated 0 with thereducing atmosphere will be effective to form a glassy film. Thus, amongthe Mn compounds, MnO is a compound specifically desirable in practice.Further it is desirable that the Mn compound have a fine granularitysuch as more than 100 meshes per inch. Such a Mn compound has an act torefine compounds contained in the silicon steel such as S and A1 duringthe box-annealing and also has an advantage of not deteriorating themagnetic property of the silicon steel sheet. Even in case the Mgcompound and Mn compound are not mixed together but the steel sheet iscoated first with the Mn compound as an under layer and then with the Mgcompound thereon so as to form double layers, a favorable glassy filmwill be obtained. In such case, the ratio of the coated amounts of theunder layer and the upper layer of the coating is so made that the ratioof MgOzMnO in the separator after drying will be in the above mentionedrange the same as in the case of mixing them.

The film forming and annealing separator may be applied by such generalmeans as a roller or spray. However, specifically, in the case ofcoating the steel sheet with the Mn compound as an under layer, thesteel sheet should be dipped in an aqueous solution such as a nitrate or4 sulfate of Mn for electrodeposition, then the oxide or hydroxide of Mncan be securely and uniformly deposited on the surface of the steel andtherefore a better glassy film can be obtained.

The above explained glassy film may be well utilized, as it is, forlaminated core materials. However, the electric insulation andanticorrosion properties can be improved if the surface is furthercoated, for example, with an inorganic film made by applying, drying orbaking a solution prepared by adding at least one part of Water glass,magnesium oxide, calcium oxide, zinc oxide and silicic anhydride to anaqueous solution containing 5 to 40 wt. percent phosphoric acid, 1 to 10wt. percent of a compound containing hexavalent chromium as chromicanhydride or dichromic acid and less than 8 wt. percent of boric acid orborate, or with an inorganic film made by applying, drying or baking anaqueous solution prepared by adding 1 to 4 wt. percent glycerine and 2to 5 wt. percent boric acid to an aqueous solution of 10 to 60 wt.percent zinc dichromate. Therefore, after a glassy insulaing film isformed by the present invention, it may be further coated with a film ofone or more of such inorganic compounds as P, Cr and Si. Further, incase the steel sheet is to be used without any stress relievingannealing, the heat resistance of the film will not be required andtherefore the glassy film can be coated thereon with an organic film ora mixed organic and inorganic film.

As above mentioned, for a film forming agent and annealing separator, amixture of a Mg compound and a Mn compound has been proved to beexcellent. However, even when using any one or more of Zn, Cu and Crcompounds instead of the Mn compound, a glassy film can also beobtained.

The mixing ratio of such Zn, Cu or Cr compound to the Mg compound is,however, in the same range as that of MgOzMnO.

EXAMPLE 1 In the production of a silicon steel sheet (with the range ofabout 2.0 to 4.0 wt. percent Si and about 0.010 to 0.040 wt. percent Aland for whose particulars U.S.P. 3,159,511 should be referred to)containing 0.017 wt. percent of total Al and 3 wt. percent Si, and whichwas subjected to a cold-rolling with a reduction rate of 65 to afterbeing hot-rolled, the steel sheet was in order to be decarburized at 800C. (by using a range of about 750 to 950 C.) for 3 minutes in anatmosphere of a decomposed ammonia gas having a dew point of 65 C., wasthinly coated on the surface with a suspension in which the mixing ratioof an annealing separator was g. of MgO, 10 g. of MnO and 1 liter of H 0(MgOzMnO: 91.7 :8.3) so that the coated amount of the annealingseparator after the drying was 8 g./m. was dried and was then finalannealed at 1200 C. for 10 hours in a reducing atmosphere (for which wasused a range of 1000 to 1200 C.) by introducing a dry mixture gas of Hand N (of a dew point having about 30 C.) for the final annealingatmosphere to produce an electric insulating film by the reaction of thesurface layer containing A1 0 produced during the annealing with theannealing separator.

The results as compared with those of the case of applying only MgO wereas follows:

Interlayer resistance (ASTM Process No, 2) in 9 cm]! sheet (35 kg./cm.

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

During the process of producing a silicon steel sheet (with the range of0.025 to 0.085 wt. percent C, 2.5 to 4.0

wt. percent Si, 0.010 to 0.065 wt. percent of acid-soluble Al and 0.005to 0.050 wt. percent SiO and for whose particulars U.S.P. 3,287,183should be referred to) contain ing 0.028 percent sol. Al and 3% Si, thesteel sheet was annealed for a short time of .5 minutes at 850 C. in anatmosphere of a decomposed ammonia gas having a dew point of 60 C. so asto make a surface layer containing A1 was thinly uniformly coated on thesurface with a suspension consisting of 70 g. of MgO, 30 g. of MnO and 1liter of H 0 (MgO:MnO:74.l:25.9) so that the coated amount of theannealing separator after the drying was 6 g./m. was dried and was thenfinal annealed at 1200 C. for 20 hours in a reducing atmosphere byintroducing a dry H gas (of dew point of about 30 C.) so that anelectric insulating film was produced.

The results as compared with those of the case of MgO only were asfollows:

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

Substantially no film. The film did not peel off.

MgO only MgO+MnO2 EXAMPLE 3 During the process (the same as in Example2) of producing a silicon steel sheet containing 0.028 percent sol. Aland 3% Si, the steel sheet was annealed for a short time of minutes at900 C. in an atmosphere of a decomposed ammonia gas of a dew point of 60C. so as to make a surface layer containing A1 0 was thinly uniformlycoated on the surface with a suspension consisting of 85 g. of MgO, g.of MnO and 1 liter of H 0 (MgO:MnO:87.4:12.6) mixing ratio so that thecoated amount of the annealing separator after the drying was 6.5 g./m.was dried, was then final annealed at 1200 C. for hours in a dry mixturegas of H and N (of a dew point of about C.) so that a glassy film wasproduced. After the residual annealing separator was removed, the steelsheet was again coated on the coated film with an equeous solutionconsisting of 20 g. of H PO 3.5 g. of MgO, 4.5 g. of CrO 2.5 g. of H BOand 100 cc. of H 0 in the mixing ratio so that the coated amount of thefilm after the baking was 3 g./cm. and was baked so that an electricinsulating film was formed. The results as compared with those of thecase of MgO only were as follows:

15. l The film peeled off greatly. 100 The film did not peel off.

MgO only MgO-i-MnOz EXAMPLE 4 During the process of producing a siliconsteel sheet (with the range of 2.0 to 4.0 wt. percent Si and 0.010 to0.090 wt. percent of acid-soluble Al) containing 0.045% of sol. Al and29% Si, and subjecting it to cold-rolling with a reduction rate of 60 to90% in thickness after hotrolling it, the sheet was annealed to bedecarburized at 800 C. (for which could be used a range of 700 to 900C.) for 3 minutes in an atmosphere of a decomposed ammonia gas of a dewpoint of 70 C. so as to make a surface layer containing Al O was thinlyuniformly coated on the surface with a suspension consisting of 30 g. ofMgO, 50 g. of MnO and 1 liter of H 0 (MgOzMnO: 42.4:57.6) mixing ratioso that the coated amount of the separator after the drying was 7 g./m.was then dried and then final annealed at 1200 C. (for which atemperature above 900 C. could be used) for 15 hours in a dry H gasatmosphere (of a dew point of 30 C.) so that a glassy film was formed.Needless to say, A1 0 produced not only in the short time annealing butalso during the final annealing reacted. Thereafter, the residualannealing separator was removed. The steel sheet was then coated with anaqueous solution consisting of 1.5 g. of ZnCr O 1.5 g. of H BO 1.5 g. ofglycerine and cc. of H 0 in the recited mixing ratio so that the coatedamount of the film after the baking was 2.5 g./m. and was baked so thatan electric insulating film was produced. The results as compared withthose of the case in which MgO only was used were as follows:

Adhesion (when bent by degrees with a diameter of 20 mm.)

MgO only MgO +MnO z EXAMPLE 5 During the process of producing a siliconsteel sheet containing 0.025 percent sol. Al and 3% Si according toU.S.P. 3,287,183, the steel sheet was annealed for a short time of 4minutes at 850 C. in an atmosphere of a dew point of 65 C., was thinlyuniformly coated on the surface with a suspension in which the annealingseparator consisted of 80 g. of MgO, 20 g. of ZnO and 1 liter of H 0 inthe recited mixing ratio so that the coated amount of separator afterthe drying was 6 g./m. was dried and was then final annealed at 1200 C.for 200 hours by introducing a dry H gas (of dew point of about 30 C.)so that a glassy electric insulating film was produced. The results ascompared with the case of MgO only were as follows:

Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

Substantially no film. Th1; film did not peel Interlayer resistance(ASTM Process No. 2) in 90111. sheet (35 kgJcmJ) MgO only MgO+ZnOEXAMPLE '6 The steel sheet in Example 5 was treated under the sameconditions as in Example 5 by using a suspension consisting of 80 g. ofMgO, 20 g. of CuO and 1 liter of H 0 instead of the MgO-ZnO seriessuspension.

EXAMPLE 7 The steel sheet in Example 5 was treated under the sameconditions as in Example 5 by using a suspension consisting of 80 g. ofMgO, 20 g. of G0,, and 1 liter of H 0 instead of the MgO-ZnO seriessuspension.

The results of the above examples as compared with those of the case ofMgO only were as follows:

Interlayer resistance (ASTM Process No. 2) in item?! sheet (35 kgJemJ)Adhesion (when bent by 180 degrees with a diameter of 20 mm.)

MgO only 0. 49 Substantially no film. Example 6 4.12 Th1? film did notpeel o Example 7 7. 27 Th5?f film did not peel EXAMPLE 8 H and wasdried. The coated amount of the Mg compound after the drying was 6.5g./m. (MgO:MnO=about 83:17). The steel sheet was then final annealed at1200 C. for hours by introducing a dry H gas (of a dew point of aboutC.) so that an electric insulating film was produced. This film was auniform glassy film having a mirror-like luster and was higher in theluster, uniformity, adhesion and electric insulation than in the casewhere MgO and MnO were used as mixed. The results as compared with thecase of MgO only were as follows:

Intcrlayer resistance Adhesion (when (ASTM Process bent by 180 N0. 2) inncmfi/ degrees with a sheet kg./cm. diameter of 20 mm.)

0. 49 Substantially no film.

Mn compound (in the under layer) plus MgO (in the upper layer).

EXAMPLE 9 When the steel sheet Was painted with 1.5 g./m. of MnO byspraying instead of the electrodeposition of the Mn compound in Example8 and was then treated the same as in Example 8, the same results as inExample 8 were obtained.

Having thus described the invention, what is claimed is:

1. A method of producing an electric insulating film on the surface ofan Al-containing silicon steel sheet con taining soluble Al in an amountof 0.01 to 0.09% by weight, comprising the steps of applying to thesurface of the silicon steel sheet a compound selected from the groupconsisting of oxides, peroxides, hydroxides and carbonates and whichproduces MnO upon heating to form an under coating layer of saidcompound, applying an upper layer of MgO to said under coating layer inan amount so that the ratio of MgO to MnO is from 98:2 to 20:80, andannealing the thus coated steel sheet at a temperature of above 1000 C.

2. A method of keeping Al-containing silicon steel sheets containingsoluble Al in an amount of from 0.01 to 0.09% by weight'separate duringfinal annealing while simultaneously producing an electric insulatingfilm on the surface of the sheets, comprising the steps of applying tothe surface of the silicon steel sheets which have a surface layer of A10 thereon at least partly formed during a prior annealing a mixture ofat least one Mg compound selected from the group consisting of oxides,peroxides, hydroxides and carbonates thereof and which produces MgO uponheating, and at least one further compound taken from the groupconsisting of Mn, Zn, Cu and Cr compounds which produce oxides of saidfurther compounds when heated, the mixing ratio of the MgO to the oxideof the further compound being from a ratio of 98:2 to a ratio of 20:80,stacking the sheets, and further annealing the stacked sheets in areducing atmosphere at a temperature of more than 1000 C. for causingthe thus obtained mixture to react with the surface layer of the steelsheet so as to form a glassy electric insulating film.

3. A method of producing an electric insulating film on the surface ofan Al-containing silicon steel sheet containing soluble Al in an amountof from 0.01 to 0.09% by weight, comprising the steps of applying to thesurface of the silicon steel sheet which has a surface layer of A1 0thereon at least partly formed during a prior annealing a mixture of atleast one Mg compound selected from the group consisting of oxides,peroxides, hydroxides and carbonates thereof and which produces MgO'upon heating, and at least one further compound taken from the groupconsisting of Mn, Zn, Cu and Cr compounds which produce oxides of saidfurther compounds when heated, the mixing ratio of the MgO to the oxideof the further compound being from a ratio of 98:2 to a ratio of 20:80,and causing the thus obtained mixture to react with the surface layer ofthe steel sheet in a reducing atmosphere of more than 1000 C. so as toform a glassy electric insulating film.

4. The method according to claim 1 wherein the step of applying themixture comprises applying the Mn compound as an under layer on thesurface of the Al-containing silicon steel sheet and then applying theMg compound as an upper layer over the Mn compound.

5. A method as claimed in claim 1 in which said further compound is a Mncompound.

6. The method as claimed in claim 5 in which said ratio of MgO to M110is in the range of :30 to :10.

7. The method as claimed in claim 5, wherein the mixture furtherincludes at least one compound from the group consisting of a Zncompound, a Cu compound and a Cr compound, which are in the form of acompound taken from the group consisting of oxides, peroxides,hydroxides and carbonates of the respective elements and produce oxidesof Zn, Cu and Cr upon heating.

References Cited UNITED STATES PATENTS 3,051,589 8/1962 Sanford et a1106-48 X 3,105,781 10/1963 Walter 148l13 X 3,132,056 5/1964 McQuade148113 X 3,151,000 9/1964 Schmidt et al 148-113 X 3,207,636 9/1965 Wadaet al 10648 X 3,259,526 7/1966 Walker et a1. l486.35 3,368,712 2/1968Sanford et al 10648 X ALFRED L. LEAVITT, Primary Examiner C. K.WEIFFENBACH, Assistant Examiner US. Cl. X.R.

